In printing with a recording apparatus in which an ink jet printing technique is used, ink is ejected from nozzles and then adheres to a recording medium. Since the nozzles are positioned away from a recording medium, printing can be carried out on surfaces having irregular shapes, such as a curved surface and an uneven surface, in a good manner. Hence, such printing has been expected to be widely used in industrial applications.
In general, inks used in such ink jet recording include aqueous dye inks containing water as the prime solvent and non-aqueous (oil-based) dye inks containing organic solvents as the prime solvent. Typical aqueous dye inks have, however, problems when they are used for industrial purposes, such as an insufficient drying rate of a printed film formed thereof on materials which are less likely to absorb liquid, e.g., plastic films; insufficient adhesive properties (adhesiveness) of a printed film; and insufficient durability, e.g., abrasion resistance, water resistance, and light fastness. On the other hand, oil-based dye inks contain chromium complex dyes composed of heavy metals such as chromium and therefore have problems in terms of safety. In order to overcome such problems related to colorants, various inks have been proposed, such as aqueous pigment inks and oil-based pigment inks which contain pigments as colorants and active-energy-ray-curable ink jet recording inks which are substantially free from non-polymerizable solvents such as organic solvents used for dilution and dissolution and which enables a printed film to be cured and dried by being irradiated with an active energy ray such as ultraviolet light.
In the case of using active-energy-ray-curable ink jet recording inks, since a printed film is irradiated with an active energy ray for curing, relatively high durability can be imparted to the printed film. There has been, however, a problem in which adhesiveness is insufficient in printing on materials which are less likely to absorb liquid, e.g., plastic films. Although a variety of oligomers and adhesive resins may be effectively used to enhance the adhesiveness, this approach leads to an increase in the viscosity of the ink, which forces printing heads to be driven under limited conditions for stable ink ejection. In particular, in the case where small ink droplets are ejected, ejection of the small droplets of a highly viscous ink causes problems such as an increase in the number of satellite droplets; a reduction in the precision of landing of ink, e.g., ejection in an unintended direction; and nozzle clogging. Use of reactive monomers having a high solubility, such as tetrahydrofurfuryl acrylate and cyclohexyl acrylate, is another effective approach to enhance the adhesiveness; however, such an approach causes a problem, for example, in terms of VOC and thus is environmentally unsuitable.
An active-energy-ray-curable ink jet ink composition (for instance, see Patent Literature 1) is known as an technique for enhancing adhesiveness; the ink composition contains polymerizable monofunctional monomers in an amount of 60% to 98% relative to the total amount of polymerizable monomers, one of the monofunctional monomers is isobornyl acrylate, the isobornyl acrylate content is from 25% to 65% relative to the total amount of the polymerizable monomers, one of the monofunctional monomers is N-vinylcaprolactam, and the N-vinylcaprolactam content is from 12.5% to 60% relative to the total amount of the polymerizable monomers.
Use of a light-emitting diode lamp (hereinafter referred to as “LED lamp”) having a low energy as a light source for active-energy-ray-curable ink jet recording inks has become popular instead of use of typical light sources such as a metal halide lamp and a high-pressure mercury lamp. A light-emitting diode UV-LED which is an example of LED lamps emits ultraviolet light having the peak emission wavelength ranging from 350 to 420 nm. In the case where the UV-LED is used for inks suitable for traditional light sources such as a metal halide lamp and a high-pressure mercury lamp, a photopolymerization initiator which can absorb light having a wavelength of around 350 to 420 nm needs to be used; however, since a pigment itself contained in the ink can absorb light having a wavelength ranging from 350 to 420 nm, even use of the photopolymerization initiator which can absorb light having a wavelength of 350 to 420 nm eventually leads to a problem of insufficient curing in many cases. Thus, in an attempt to cure the inks disclosed in Patent Literatures 1 and 2 with an LED lamp, durability, particularly scratch resistance, cannot be imparted to a printed film in some cases.